In an electronic device in which a plurality of conductor planes are present, for example, a current flowing into a circuit during switching of a digital circuit induces a magnetic field and a voltage fluctuation caused during switching induces an electric field, so that electromagnetic waves are generated. The electromagnetic waves become electromagnetic noise propagating through a parallel flat line formed of conductor planes. The electromagnetic noise causes problems such as instability of operation of other circuits and deterioration in radio performance of the device. Accordingly, if it is possible to establish a technique for suppressing the electromagnetic noise, the stability of circuits and the radio performance of the device can be improved.
Examples of a conventional technique employed to suppress the electromagnetic noise include a method of disposing a decoupling capacitor between conductor planes. However, the conventional technique has the following problem.
In the case of a technique using a decoupling capacitor, an inevitable parasitic inductance of a capacitor makes it difficult to set a self resonant frequency to a high frequency of several hundred MHz. Accordingly, in general, the technique using a decoupling capacitor can be applied to a frequency of at most about several hundred MHz, while it cannot be applied to a high frequency band used for radio communication in recent years (for example, frequency bands of 2.4 GHz and 5.2 GHz which are used for wireless LAN, and frequency bands of 1.8 GHz, 2.6 GHz, and 3.5 GHz which are used for LTE (Long Term Evolution).
As techniques for solving the above-mentioned problem, techniques using a structure are disclosed in, for example, the Specification of U.S. Pat. No. 7,215,007 (PTL 1), Japanese Unexamined Patent Application Publication No. 2010-199881 (PTL 2), and Japanese Unexamined Patent Application Publication No. 2010-10183 (PTL 3). The structure disclosed in Patent Literature 1, 2, and 3 is a structure having EBG (Electromagnetic Band Gap) characteristics having a dispersion relation with a band gap for inhibiting propagation of electromagnetic waves at a characteristic frequency (this structure is hereinafter referred to as an EBG structure). This structure can suppress propagation of electromagnetic noise between power supply planes. The use of the technique using the EPG structure makes it possible to obtain an effect of suppressing electromagnetic noise in a high frequency band of GHz.